The present invention relates to a device with at least one storage container for material to be treated, preferably for plastic material granules, with at least one inlet line and at least one return line for process air and with at least one heating device for the process air and at least one blower for conveying the process air.
In certain applications in the drying technology, it is necessary to dry material resulting from a manufacturing process to a certain defined residual moisture contents. For a plastic material in granular form, it is, for example, possible to achieve a minimum residual moisture contents of 0.01%. Depending on the production conditions of the plastic material, the residual moisture contents may vary upon delivery, for example, between 0.02% and 1.8%. For further processing of the plastic material granules, it is however necessary to reduce the moisture contents to, for example, a value of exactly 0.05% to at most 0.06% in order to achieve certain material properties of the finished part produced from the plastic material granules.
In conventional devices process air is generated with a dew point of approximately -20.degree. C. to -50.degree. C. The process air is guided through the material to be treated and takes up moisture while passing through the material. The return air flowing out of the storage container is introduced into a drying agent cartridge which takes up the moisture contained in the process air so that dry process air can be returned into the storage container. The dew point is subject to fluctuations because the drying agent cartridge will be saturated over the course of its period of use which is, in general, approximately 2 to 5 hours. Thus, the drying capacity will be reduced over time. FIG. 3 shows this relationship in a diagram in which the dew point is represented as a function of the drying time T. At the beginning of the drying process the dew point is very low; in the represented embodiment it is below -50.degree. C. With increasing treatment time, respectively, drying time the drying cartridge is loaded increasingly with moisture so that it can take up less and less moisture from the process air. This results in an increase of the dew point until the drying agent cartridge is saturated and must be replaced. The drying agent cycle of the drying agent cartridge is represented in FIG. 3 and identified with I. When a new drying agent cartridge is introduced, a new drying cycle II begins. Since the new drying agent cartridge has not taken up any moisture yet, the dew point is suddenly reduced and is again very low. With increasing process time, the dew point climbs again to the system-specific maximum dew point which is, in general, approximately -20.degree. C. Upon reaching this maximum value, the drying agent cartridge is again replaced with a fresh, regenerated drying agent cartridge. A new drying cycle III begins. In this manner a dew point curve 34, as a function of the degree of saturation of the drying agent cartridge, results for each drying process.
Attempts have been made to smoothen this curve. This is represented by the curve 35 in FIG. 3. In this case, drying agent dampers are introduced into the system which are in the form of molecular sieve cartridges. Thus, the dew point curve 35 is smoother than the dew point curve 34. It is also possible to reduce the degree of fluctuation of the dew point over the course of the process with a multi-cartridge technique.
It is furthermore known to realize a continuously low dew point with drying agent wheels. However, the dew point will fluctuate with the moisture load of the system. Thus, in the past the material to be treated was subjected to drying air with a dew point as low as possible for a certain amount of time in order to achieve a minimal residual moisture contents as low as possible. However, since the initial moisture contents of the material fluctuates, this drying technique cannot provide any improvement. Thus, the drying parameters are to be adjusted for the worst case scenario. However, for good drying conditions, this will result in an overdrying of the material leading to unfavorable processing parameters and material properties.
It is therefore an object of the present invention to improve a device of the aforementioned kind such that, for controlling in a directed manner a defined residual moisture contents of the material to be dried, a preselected dew point temperature can be maintained constant at least within certain limits.